The present invention relates to drilling techniques for oil wells, gas wells, water wells, geothermal wells, and the like. More precisely, the invention relates to cementing slurries of low density and low porosity.
After an oil well or the like has been drilled, casing or coiled tubing is lowered down the borehole and is cemented over all or part of its height. Cementing serves in particular to eliminate any fluid interchange between the various formation layers that the borehole passes through, preventing gas from rising via the annulus surrounding the casing, or indeed it serves to limit ingress of water into a well in production. Naturally, another main objective of cementing is to consolidate the borehole and to protect the casing.
While it is being prepared and then injected into the well so as to be placed in the zone that is to be cemented, the cementing slurry must present relatively low viscosity and it must have rheological properties that are practically constant. However, once it is in place, an ideal cement would rapidly develop high compression strength so as to enable other work on the well that is being built to start again quickly, and in particular. so as to enable drilling to be continued.
The density of the cement must be adjusted so that the pressure at the bottom of the well compensates at least for the pore pressure in the geological formations through which the well passes so as to avoid any risk of eruption. As well as this lower limit on density, there is also an upper limit. This upper limit is that the hydrostatic pressure generated by the column of cement plus the head losses due to the circulation of the fluids being pumped must remain below the fracturing pressure of the rocks in the section being cemented. Certain geological formations are very fragile and require densities close to that of water or even lower.
The risk of eruption diminishes with column height so the density required for compensating pore pressure is then lower. In addition, cementing a large height of column is advantageous since that makes it possible to reduce the number of sections that are cemented. After a section has been cemented, drilling must be restarted at a smaller diameter, so having a large number of sections requires a hole to be drilled near the surface that is of large diameter, thereby giving rise to excess cost due to the large volume of rock to be drilled and due to the large weight of steel required for the sections of casing, given their large diameters.
All of those factors favor the use of cement slurries of very low density.
The cement slurries in the most widespread use have densities of about 1900 kg/m3, which is about twice the density desired for certain deposits. To lighten them, the simplest technique is to increase the quantity of water while adding stabilizing additives (known as xe2x80x9cextendersxe2x80x9d) to the slurry for the purpose of avoiding particles settling and/or free water forming at the surface of the slurry. Manifestly, that technique cannot get down to a density close to 1000 kg/m3. Furthermore, hardened cements formed from such slurries have greatly reduced compression strength, a high degree of permeability, and poor adhesive capacity. For these reasons, that technique cannot be used to go below densities of about 1300 kg/m3 while still conserving good isolation between geological layers and providing sufficient reinforcement for the casing.
Another technique consists in lightening the cement slurry by injecting gas into it (generally air or nitrogen) before it sets. The quantity of air or nitrogen added is such as to reach the required density. It can be such as to form a cement foam. That technique provides performance that is a little better than the preceding technique since the density of gas is lower than that of water, so less needs to be added. Nevertheless, in oil industry applications density remains limited in practice to densities greater than 1100 kg/m3, even when starting with slurries that have already been lightened with water. Above a certain xe2x80x9cquality of foamxe2x80x9d, i.e. a certain ratio of gas volume to volume of the foamed slurry, the stability of the foam falls off very quickly, the compression strength of the foam after it has set becomes too low, and its permeability becomes too high, thereby compromising durability in a hot aqueous medium which includes ions that are aggressive to a greater or lesser extent for cement.
U.S. Pat. No. 3,804,058 and GB 2,027,687A describe the use of hollow glass or ceramic micro-spheres to produce low density cement slurries for use in the oil and gas industry.
An object of the present invention is to provide cementing slurries that are more particularly adapted to cementing oil wells or the like, having both low density and low porosity, and that are obtained without introducing gas.
According to the invention, this object is achieved by a cement slurry for cementing an oil well or the like, the slurry having a density lying in the range 0.9 g/cm3 to 1.3 g/cm3, in particular in the range 0.9 g/cm3 to 1.1 g/cm3, and being constituted by a solid fraction and a liquid fraction, having porosity (volume ratio of liquid fraction over solid fraction) lying in the range 38% to 50%, and preferably less than 45%.
The solid fraction is preferably constituted by a mixture comprising:
60% to 90% (by volume) of lightweight particles having a mean size lying in the range 20 microns (xcexcm) to 350 xcexcm;
10% to 30% (by volume) of micro-cement having a mean particle diameter lying in the range 0.5 xcexcm to 5 xcexcm;
0 to 20% (by volume) of Portland cement, having particles with a mean diameter lying in the range 20 xcexcm to 50 xcexcm; and
0 to 30% (by volume) of gypsum.
The low porosities achieved make it possible to optimize mechanical properties and permeability. By presenting mechanical properties that are much better than those of conventional lightened systems, and permeabilities that are lower, the leakproofing and adhesion properties of ultralightweight cement and the resistance of such formulations to chemical attack are thus much better than with the systems presently in use for low densities, even though the invention makes it possible to reach densities that are exceptionally low, and in particular that are lower than the density of water. In addition, slurries of the invention do not require gas, thus making it possible to avoid the logistics that would otherwise be required for manufacturing foamed cements.
The method of the invention is characterized in that particulate additives are incorporated in the cement slurry, such that in combination with one another and with the other particulate components of the slurry, and in particular with the particles of micro-cement (or comparable hydraulic binder), they give rise to a grain-size distribution that significantly alters the properties of the slurry. The said particulate additives are organic or inorganic and they are selected for their low density.
The low density is obtained by combining lightweight particles and cement (or a comparable hydraulic binder). Nevertheless, Theological and mechanical properties will only be satisfactory if the size of the particles and the volume distribution thereof is selected in such a manner as to maximize the compactness of the solid mixture.
For a solid mixture having two components (the lightweight particles and the micro-cement), this maximum compactness is generally obtained for a volume ratio of lightweight particles to micro-cement lying in the range 70:30 to 85:15, and preferably in the range 75:25 to 80:20, for lightweight particles selected to be of a size that is at least 100 times approximately the size of the particles of micro-cement, i.e. in general, particles that are greater than 100 xcexcm in size. These values can vary, in particular as a function of the greater or lesser dispersion in the grain-size distribution of the lightweight particles. Particles having a mean size greater than 20 microns can also be used, but performance is not so good. Particles greater than 350 microns are generally not used because of the narrow size of the annular gaps to be cemented.
Mixtures having three or more components are preferred since they make it possible to obtain greater compactness if the mean sizes of the various components are significantly different. For example, it is possible to use a mixture of lightweight particles having a mean size of 150 microns, lightweight particles having a mean size of 30 microns, and micro-cement, at a volume ratio lying close to 55:35:10, or departing a little from these optimum proportions, the mixture being constituted by 50% to 60% (by volume) of the first lightweight particles of mean diameter lying in the range 100 xcexcm to 400 xcexcm, 30% to 45% of the second lightweight particles of mean diameter lying in the range 20 xcexcm to 40 xcexcm, and 5% to 20% of micro-cement. Depending on the application, the fraction of lightweight particles of intermediate size can be replaced by Portland cement of ordinary size, in particular class G Portland cement.
The term xe2x80x9cmicro-cementxe2x80x9d is used in the invention to designate any hydraulic binder made up of particles of mean size of about 3 xcexcm and including no, or at least no significant number of, particles of size greater than 10 xcexcm. They have a specific surface area per unit weight as determined by the air permeability test that is generally about 0.8 m2/g.
The micro-cement can essentially be constituted by Portland cement, in particular a class G Portland cement typically comprising about 65% lime, 25% silica, 4% alumina, 4% iron oxides, and less than 1% manganese oxide, or equally well by a mixture of Portland micro-cement with microslag, i.e. a mixture making use essentially of compositions made from clinker comprising 45% lime, 30% silica, 10% alumina, 1% iron oxides and 5% to 6% manganese oxide (only the principal oxides are mentioned here; and these concentrations can naturally vary slightly as a function of the supplier) . For very low temperature applications ( less than 30xc2x0 C.), Portland micro-cement is preferable over a mixture of micro-cement and slag because of its reactivity. If setting at right angles is required, plaster (gypsum) can be used for all or some of the middle-sized particles.
The lightweight particles typically have density of less than 2 g/cm3, and generally less than 0.8 g/cm3. By way of example, it is possible to use hollow microspheres, in particular of silico-aluminate, known as cenospheres, a residue that is obtained from burning coal and having a mean diameter of about 150 xcexcm. It is also possible to use synthetic materials such as hollow glass beads, and more particularly preferred are beads of sodium-calcium-borosilicate glass presenting high compression strength or indeed microspheres of a ceramic, e.g. of the silica-alumina type. These lightweight particles can also be particles of a plastics material such as beads of polypropylene.
In general, the density of the slurry is adjusted essentially as a function of which lightweight particles are chosen, but it is also possible to vary the ratio of water to solid (keeping it in the range 38% to 50% by volume), the quantity of micro-cement or of comparable hydraulic binder (in the range 10% to 30%), and adding Portland cement of ordinary size as a replacement for a portion of the lightweight particles.
Naturally, the slurry can also include one or more additives of types such as: dispersants; antifreeze; water retainers; cement setting accelerators or retarders; and/or foam stabilizers, which additives are usually added to the liquid phase, or where appropriate incorporated in the solid phase.
Formulations made in accordance with the invention have mechanical properties that are significantly better than those of foamed cements having the same density. Compression strengths are very high and porosities very low. As a result, permeabilities are smaller by several orders of magnitude than those of same-density foamed cements, thereby conferring remarkable properties of hardness on such systems.
The method of the invention considerably simplifies the cementing operation, since it avoids any need for logistics of the kind required for foaming.
Slurries prepared in accordance with the invention also have the advantage of enabling all of the characteristics of the slurry (rheology, setting time, compression strength, . . . ) to be determined in advance for the slurry as placed in the well, unlike foamed slurries where certain parameters can be measured on the slurry only prior to the introduction of gas (setting time).